With the development of technology, mobile phones, digital cameras, tablet PCs, notebook computers and other portable electronic devices become more and more popular. The portable electronic devices are intended for personal use, with certain privacy. The information stored in the portable device such as phonebook, photos, personal information, etc., are privately owned. If the electronic device is lost, these data may be accessed by other people, causing unnecessary loss. Even though there are several ways using password to protect the electronic device from being used by the other people, the password may be easily to be spread or cracked, which lowers the security. Moreover, the user needs to remember the password so as to use electronic devices. If the user forgets the password, troubles would be brought to the user. Hence, personal fingerprint identification is utilized to achieve the purpose of personal identification, for enhancing the data security.
In general, the fingerprint identification system utilizes the pixel circuit to receive a finger touch. The pixel circuit would convert a touch capacitance, formed between the pixel circuit and the finger, as the pixel output signal, and the backend circuit of the pixel circuit may determine whether the pixel circuit is corresponding to a finger ridge or a finger valley, according to the pixel output signal. In addition, the power of the pixel circuit is supplied by a charge pump. The pixel circuit receives a pixel clock signal clk_pxl so as to convert the touch capacitance into the pixel output signal. The charge pump circuit receives a charge pump clock signal clk_cp to output a supply voltage Vsupply′ to the pixel circuit. Unavoidably, the supply voltage generated by the charge pump circuit has a ripple rp′, and a waveform of the ripple rp′ is related to the charge pump clock signal clk_cp.
Generally, the pixel clock signal clk_pxl and the charge pump clock signal clk_cp do not have direct relationship in between. Under a condition that the pixel clock signal clk_pxl is independent of the charge pump clock signal clk_cp, among every sampling interval of the pixel circuit, the average of the ripple rp′ is not the same. In this case, the ripple rp′ of the charge pump circuit would bring more noise to the fingerprint identification system and reduce an accuracy of the fingerprint identification, such that an overall performance of fingerprint identification is reduced.
Therefore, how to reduce effect caused by ripple is a significant objective in the field.